Micro-holographic recording and reading apparatus

ABSTRACT

Micro-holographic Fourier transform recorder apparatus arranged and constructed so as to provide, with suitable reader apparatus reproduced images of a sufficient size for direct viewing and compensated for aberration and laser speckle. The recorder includes means for providing an enlarged image of the transparency to be recorded prior to making the hologram whereby an enlarged image is produced on reconstruction. The appearance of laser speckle is eliminated in the reader apparatus by means of an appropriate vibratory motion applied to the diffusing screen on which the image is presented.

SEARCH @CQM \J" v w Umted State; V [is] 3,704,930

McMahon SUBSTITUTF R M m 1 1 Dec. 5,1972

1541 MICRO-HOLOGRAPHIC RECORDING Bowman, 7 Applied Optics, 2280-2284, (11/1968).

AND READING APPARATUS Primary Examiner-David Schonberg [72] Inventor.llaioalgzld H. McMahon, Carl1sle, Assistant Examiner Robert L ShermanAttorney-S. C. Yeaton [73] Assignee: Sperry Rand Corporation [221 Filed:March 31,1971 [57] ABSTRACT Micro-holographic Fourier transform recorderap- [21] Appl. No.. 129,737 paratus arranged and constructed so as toprovide, with suitable reader apparatus reproduced images of a [521 u s.c1 ..3so/3.s, 353/7 Sufficient Size for direct viewing and compensatedfor [511 1111.01. ..'..G02b 27/22 aberration and laser Speckle Therecorder includes [58] Field of Search ..35o/3.s; 353/7 means forPmviding an enlarged image of the parency to be recorded prior to makingthe hologram [56] (defences Cited whereby an enlarged image is producedon reconstruction. The appearance of laser speckle is OTHER PUBLICATIONSeliminated in the reader apparatus by means of an appropriate vibratorymotion applied to the diffusing sati i;p g:'c i :8 9 9 /i' screen onwhich the image is presented. Vitols, 8 IBM Tech. Disc. Bull.,1581-1583, (4/1966). 10 Claims, 2 Drawing Figures TRANSPARENCY PATENTED5M 3.704.930 SHEEI 2 OF 2 DIFFUSING SCREEN INVENTOI? DONALD H. Ma MAHO/VAlla/(MY recordings can be erased and re-recorded MICRO-HOLOGRAPHICRECORDING AND READING APPARATUS BACKGROUND OF THE INVENTION 1. Field ofthe Invention The present invention relates to micro-holography and moreparticularly to micro-holographic recording particularly adapted toreduction of conventional microfilm or other comparably small recordingsin a manner to obtain enlarged images suitable for direct viewing uponreconstruction with compatible apparatus.

2. Description of the Prior Art For the past several decades reductionof information for storage purposes has typically been performed onmicrof lm utilizing conventional photographic imag ing techniques.Reduction ratios of about to l became more or less standard as acompromise between conveniently realizable storage densities and legibleinformation retrieval. Larger de-magnification ratios are readilyachievable, of course, for small scale operations, but for generalutility in applications involving large quantities of data the amount'of reduction is limited by various reliability factors relating to themechanical focusing tolerances of step and repeat recording techniques,unavailability of inexpensive high quality lenses, and dust or otherfilm imperfections which seriously degrade the recorded and reproducedimages.

One recently developed technique capable of storage at reduction ratiosin excess of one hundred to one utilenlarged image of the data prior tobeing focused on the holographic plate, whereby upon reconstruction ofthe recorded wavefront, in a reader apparatus, the resulting image willbe sufficiently large for'direct viewing. In further explanation of thepreceding comment, it will be appreciated that if a microfilmtransparency is directly recorded as a Fourier transform, the imageobtained on reconstruction will be of the same size as the originaltransparency unless magnified in some way. The magnification may beobtained holographically, as will be explained subsequently, but only toa certain degree if holographic aberrations are not to degrade imageresolution. Alternatively, magnifying elements such as lenses may beincorporated in the reader apparatus, but this is consideredeconomically undesirable. In any case, it will be recognized that thereis a tradeoff between storage density and the subsequent magnificationrequired to obtain reproduced images of izes an intermediate stepwherein the reduced images therefore susceptible to inspection so thatimperfect I prior to transferral to'the film.

Holography offers another means for increasing the image reductionratios and'has the advantage of providing certain beneficial featuresnot obtainable with conventional photographic method, for exampleinsensitivity to localized imperfections on the hologram. Moreover, byutilizing Fourier transform techniques the recording process can berendered insensitive to positional variations of the input informationin a plane transverse to the recording light beam. in addition, bystoring the information in the form of Fourier transforms, both thetransverse positioning and viewing suitable size for direct reading.High storage density is obtained when the transparency image subtends alarge solid angle relative to the hologram. This requires that amicrofilm size transparency be placed only a few centimeters from thehologram. An arrangement of this sort, however, introduces considerableaberration in the reconstructed images. Considerable aberration alsooccurs in the case where large holographic magnification is employed. Ingeneral, if a high storage density is achieved with direct Fouriertransform recording from the microfilm transparency, the amount ofmagnification required in the reader will be too high to be accomplishedwithout introducing intolerable aberration in the reproduced images.These problems are minimized with a recorder constructed according tothe principles of the present invention. More specifically, regardingthe structure of the recorder, the

microfilm transparency, of which a reduced size holographic recording isto be made, is positioned in the path of a converging beam which isdiffracted by the transparency to form a signal beam. A lens positionedat the focal plane of the converging beam receives the diffracted signalbeam and refracts it to form an enlarged image of the transparency atwhich location another lens ispositioned to focus the image light ontothe holographic plate in superposed relation with a coherently relatedreference beam. Rapid convergence focus are made less critical whenretrieving data for;

SUMMARY OF THE INVENTION A preferred recording apparatus of the presentinvention comprises a conventional Fourier transform holographicrecorder modified in a manner to cause the signal beam bearing data tobe recorded to form an compatible with high storage density of theenlarged image data is permissible since similar enlarged images will beproduced without holographic magnification upon reconstruction of theholographically stored data. Further holographic magnification can beproduced by altering the convergence of the reference beam. Aberrationsproduced in this instance by the holographic magnification are reducedwhen the enlarged image is located more remote from the hologram. Thedata from a plurality of microfilm transparencies are recorded side byside in a similar fashion to obtain a two-dimensional holographic array.

A real image of the data contained in any selected hologram of the arraymay be simply produced in a reader apparatus by illuminating thehologram with an identical reference beam directed oppositely to thatused for recording. The read image appears at the location of theenlarged image plane of the recording apparatus, is the same size as therecorded image, and is presented on a diffuser screen positionedthereat. A

Fresnel lens positioned immediately in front of the diffuser in the pathof the reconstructed light wave enhances the image brightness andcontrast. ln addition to the foregoing components, the reader alsoincludes a mechanism for dithering the diffuser screen in a directionparallel to its major surfaces for eliminating the appearance of laserspeckle and thereby providing for increased legibility and morecomfortable viewing.

Other innovative features of the recorder and reader apparatus will bediscussed in the subsequent detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic side viewillustration of a preferred micro-holographic recorder apparatusembodying the principles of the present invention.

FIG. 2 18 a schematic side view representation of a reader apparatusconstructed in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODlMENT Referring to FIG. 1, therecording apparatuscomprises a laser which emits a light beam 11directed toward beam splitter 12 where it is divided into reference andsignal beams 13 and 14, respectively. The reference beam propagatesthrough converging lens 16, reflects from mirror 17 and passes throughthe aperture in iris 18 onto the photosensitive surface of holographicfilm plate 19. The signal beam meantime reflects from mirror 21 ontolens 22 which focuses the beam through a central pin hole aperture inplate 23. Lens 22 and the aperture in plate 23 function cooperatively toprovide a point source of light compensated for imperfections in theoptical components disposed in the path of the light beam up to thispoint. Lens 24 collects andcollimates the light emanating from the pinhole aperture while'lens 26 in turn converges the collimated light to apoint located near the center of lens 27. A' microfilm transparency,designated by arrow 28, is positioned in the path of the convergingsignal beam adjacent lens 26' and beyond the front focal plane of lens27 whereby an enlarged image of the transparency,

cation of holographic lens 31. The stop 33 should be positioned at thefocal point of lens-26; positioning of lens 27 is not as critical. Anyordinary converging lens could be used in place of the holographic lensor in combination therewith but a simple inexpensive hologram lens isgenerally suitable when used with monochromatic laser light for theintended purpose of rapidly converging the image light through iris 18to focus on the holographic film in superposed relation with thereference beam incident thereon. Interference between the signal andreference beams thus produces a Fourier transform hologramrepresentative of the information contained on the microfilmtransparency. For best results with regard to resolution of the imagesreproduced from the recordings, it is advisable to arrange for theintensities of the signal and reference beams in the informationcontaining portion of the hologram to be approximately equal uponstriking the holographic plate.

lt will be noted that a Fourier transform of the transparency data isalso formed at the focal plane of lens 26, that is, at the location ofzero order stop 33. Consequently, the Fourier transform hologramrepresentative of the transparency could have been produced simply bylocating the holographic film at the position of lens 27 and directingthe reference beam to interfere with the signal beam at that point. Ifthe recording had been made in this manner. however, it will beappreciated that the image produced upon reconstruction would be of thesame size as the original microfilm transparency, about three centimetersquared, which is unsatisfactory for reading a journal size page. Somedegree of magnification could be achieved, of course, with conventionallenses or by means of holographic techniques but not in an amountsufficient for practical usewithout introducing severe aberrations. Inany event, by inserting lens 27 at the location of the diffractionpattern of the transparency. an enlarged image 29 is provided which.provided the convergence of the reference beam is unaltered, will bereproduced upon reconstruction of the wavefront data stored in theholographic film. High quality images may be obtained in this way insizes on the order of about four by five inches as compared to the A by2 centimeter dimensions of the microfilm transparency. Moreover, byinserting the pin point stop 33 to block the undiffracted light from thetransparency, a considerable improvement in the contrast of the image isrealized. Recording of a multiplicity of pages is accomplished in theforegoing manner simplyvby moving the holographic plate to position adiscrete area behind the opening in the iris for each recording.

It will also be noted that the reference beam is slightlydivergent whenit strikes the holographic plate. This permits some degree ofmagnification to be achieved in the reproduced image simply by using acollimated reference beam in the reader apparatus. Specifically by wayof example, if the focal point 16' of lens 16 is arranged to be spacedfrom the holographic plate by a distance equal to twice the spacingbetween the holographic plate and image 29, a two to one magnificationwill be obtained in a reader which uses a collimated reference beam.

Reproduction of data stored holographically in the above describedmanner is accomplished with the reader apparatus of FIG. 2. A collimatedreference beam 34 obtained from laser 35 and directed to impinge onholographic plate 19 on the side opposite to that used in recordingcauses the signal wavefront stored therein to be reconstructed whereuponan image, corresponding to the enlarged image of the recorder, isproduced at the location of diffusing screen 36 which is spaced twicethe distance from the hologram as was the recorder image 29. Thereproduced image is twice the size of the enlarged recorder image 29based on the previously mentioned spacings of the various components,and in the case of a 4 inch by 5 inch image at the hologram lens of therecorder will result in an 8 inch by 10 inch reader image, a convenientsize for easy viewing. The two to one holographic magnification obtainedby altering the convergence '(or divergence) of the reproducingreference beam has been found quite satisfactory since only a smalldegree of aberration occurs under these conditions.

- A Fresnel lens 37 positioned in the path of the reconstructedwavefront immediately in front of the diffusing screen focuses the lightenergy through the screen thereby enhancing the brightness and assuringuniform intensity of all parts of the readout image. In the absence ofthe Fresnel lens, the image light intensity would be seriously degradedin the case of an isotropic diffuser which scatters incident lightfairly uniformly in all directions whereas for an anisotropic diffuserwhich concentrates the light in a cone of predetermined angle, the imagebrightness would be preserved but would tend to diminish if the viewer'seye moved slightly beyond the edge of the cone.

Positional tolerances are not critical in the reader. It is necessarymerely to assure that the reference beam strikes only one hologram ofthe two dimensional array at any given time. Preferably, to yieldmaximum resolution of the image, the laser provided the reference beamshould be operated ina single mode and at the same wavelength as usedfor recording.

Another problem which must be considered in the reader apparatus is thatof laser speckle, a characteristic grainy pattern which causesconsiderable annoyance to the viewer by making the image patternindistinct. Laser speckle is caused by random constructive ordestructive interference of components of laser light from pointslocated so close together on the diffusing screen as to be unresolvableby the eye. The speckle pattern can be broken up and appear to beeliminated by imparting motion to the diffusing screen so that the eyeintegrates the image pattern over a period of time. ln the case of adiffuser consisting of troughs approximately I micronwide, the specklecan be eliminated if the screen is moved through this distance in lessthan one-fiftieth of a second. Hence, a screen motion of approximatelyone-tenth of a centimeter per second will generally be sufficient toeliminate speckle, although in some cases a higher rate may be requiredfor one reason or another. This is accomplished in the reader apparatusby means of a vibratory mechanism 38 comprising a conventional cam andlinkage arrangement coupled to the diffusion screen to cause it tooscillate up and down in a plane parallel to its major surfaces throughan excursion of about one-half inch.

A few additional features of the recorder remain to I be discussed. Asindicated in FIG. 1, the holographic converging lens 31 is constructedso that its focal point is equidistant from each edge of the lens. Thisconstruction, although not essential, is nevertheless desirable since itminimizes field curvature in the reconstructed images. If the hologramlens is constructed to minimize field curvature, it must then be cantedwith respect to the optical axis of the incident signal beam because ofthe inherent nature of a hologram to produce a first order diffractionlobe displaced to the side of the optical axis. Once the foregoingconditions are established it also becomes desirable to cant themicrofilm transparency with respect to the optical axis in a directionopposite to that of the hologram lens so that the enlarged transparencyimage lies in the same plane as the hologram lens. This arrangement willminimize distortion in the reconstructed images.

Regarding the microfilm transparencies, it is preferably that these benegatives wherein the information is presented by transparent regions ofhigh transmissivity against an opaque background. This enablessubstantially all of the available light to appear in the informationregions with very little light in the background of the reproducedimages. Recordings can also be made from positive transparencies, but inthat case the light will be concentrated in the background of thereproduced images thus causing the light to be distributed over aconsiderably larger area and thereby significantly reduce imagebrightness and contrast.

Finally, the preferred orientation of the reference beam and theholographic plate normal relative to the orientation of the transparencymodulating the signal beam should be noted. From an analysis ofholographic aberrations it can be shown that less astigmatism will occurin the reproduced images if the recorder is arranged so that the planedefined by the hologram normal and the reference beam is alignedparallel with the larger dimension of the enlarged image. Thus in thecase of a 4 inch by 5 inch enlarged image, if the 4 inch dimension isperpendicular to the plane of the figure and the 5 inch dimension is inthe plane of the figure, the reference beam should be directed in theplane of the figure so that it strikes the holographic plate from aboveor below the transparency image rather than from the side, with respectto the signal beam.

While the invention has been described in its preferred embodiment, itis to be understood that the words which have been used are words ofdescription rather than limitation and that changes may be made withinthe purview of the appended claims without departing from the truespirit and scope of the invention in its broader aspects.

lclaim:

l. A micro-holographic recording apparatus comprising light source meansincluding a laser for directing a light beam through'a transparencycontaining data which is to be recorded,

first light focusing means positioned to collect light transmittedthrough the transparency and produce an enlarged image of thetransparency,

a holographic recording plate,

second light focusing means positioned at the location of the enlargedimage for converging the enlarged image onto said holographic recordingplate, and

means for directing a coherently related reference beam onto saidholographic recording plate to interfere with the converged image lightto produce a hologram of the transparency data.

2. The apparatus of claim 1 including means positioned in the path ofthe light transmitted through the transparency for blocking undiffractedlight transmitted therethrough.

3. The apparatus of claim 1 wherein the enlarged image is of generallyrectangular configuration and the plane of incidence of the coherentlyrelated beam on said holographic recording plate is alignedsubstantially parallel to the longer dimension of the enlarged image.

4. The apparatus of claim 1 wherein said second light focusing meanscomprises a holographic lens having a focal point equidistant fromopposite edges of said holographic lens and is oriented so as to becanted with respect to the optical axis of the beam forming the enlargedimage.

5. The apparatus of claim 4 including means for positioning thetransparency so that it is canted with respect to the optical axis ofthe beam directed through the transparency so as to form the enlargedimage in a plane superposed with said holographic lens.

6. The apparatus of claim 5 including means positioned in the path ofthe light transmitted through the transparency for blocking light whichis not diffracted by the transparency.

7. The apparatus of claim 5 wherein the enlarged image is 0; generallyrectangular configuration and the plane of incidence of the coherentlyrelated beam on said holographic recording plate is aligned parallel tothe longer dimension of the enlarged image. 7

8. The apparatus of claim 7 including meanspositioned'in the path of thelight transmitted through the transparency for blocking light which isnot diffracted a microfilm transparency containing data to be recorded,

light source means including a laser for directing a converging lightbeam through said transparency,

first light focusing means located proximate the focal point of theconverging beam for collecting the light transmitted through saidtransparency and producing an enlarged image thereof,

second light focusing means positioned at the location of said enlargedimage for converging the light contained therein.

a holographic plate positioned at the focal plane of said second lightfocusing means for receiving said converging image light, and

means for directing a coherently related reference beam onto saidholographic plate to interfere therein with said converging image lightto produce a hologram representative of the transparency data.

10. The apparatus of claim 9 wherein said second light focusing meanscomprises a holographic lens having a focal point equidistant fromopposite edges of said holographic lens and is oriented so as to becanted with respect to the optical axis of the beam forming the enlargedimage, and further including means for positioning the transparency sothat it is canted with respect to the optical axis of the convergingbeam directed 'through the transparency so as to form the enlarged imagein a plane superposed with said holographic lens.

* it i k

1. A micro-holographic recording apparatus comprising light source meansincluding a laser for directing a light beam through a transparencycontaining data which is to be recorded, first light focusing meaNspositioned to collect light transmitted through the transparency andproduce an enlarged image of the transparency, a holographic recordingplate, second light focusing means positioned at the location of theenlarged image for converging the enlarged image onto said holographicrecording plate, and means for directing a coherently related referencebeam onto said holographic recording plate to interfere with theconverged image light to produce a hologram of the transparency data. 2.The apparatus of claim 1 including means positioned in the path of thelight transmitted through the transparency for blocking undiffractedlight transmitted therethrough.
 3. The apparatus of claim 1 wherein theenlarged image is of generally rectangular configuration and the planeof incidence of the coherently related beam on said holographicrecording plate is aligned substantially parallel to the longerdimension of the enlarged image.
 4. The apparatus of claim 1 whereinsaid second light focusing means comprises a holographic lens having afocal point equidistant from opposite edges of said holographic lens andis oriented so as to be canted with respect to the optical axis of thebeam forming the enlarged image.
 5. The apparatus of claim 4 includingmeans for positioning the transparency so that it is canted with respectto the optical axis of the beam directed through the transparency so asto form the enlarged image in a plane superposed with said holographiclens.
 6. The apparatus of claim 5 including means positioned in the pathof the light transmitted through the transparency for blocking lightwhich is not diffracted by the transparency.
 7. The apparatus of claim 5wherein the enlarged image is of generally rectangular configuration andthe plane of incidence of the coherently related beam on saidholographic recording plate is aligned parallel to the longer dimensionof the enlarged image.
 8. The apparatus of claim 7 including meanspositioned in the path of the light transmitted through the transparencyfor blocking light which is not diffracted by the transparency.
 9. Amicro-holographic recording apparatus comprising a microfilmtransparency containing data to be recorded, light source meansincluding a laser for directing a converging light beam through saidtransparency, first light focusing means located proximate the focalpoint of the converging beam for collecting the light transmittedthrough said transparency and producing an enlarged image thereof,second light focusing means positioned at the location of said enlargedimage for converging the light contained therein, a holographic platepositioned at the focal plane of said second light focusing means forreceiving said converging image light, and means for directing acoherently related reference beam onto said holographic plate tointerfere therein with said converging image light to produce a hologramrepresentative of the transparency data.
 10. The apparatus of claim 9wherein said second light focusing means comprises a holographic lenshaving a focal point equidistant from opposite edges of said holographiclens and is oriented so as to be canted with respect to the optical axisof the beam forming the enlarged image, and further including means forpositioning the transparency so that it is canted with respect to theoptical axis of the converging beam directed through the transparency soas to form the enlarged image in a plane superposed with saidholographic lens.